Circuit configurations in half-bridge circuit and full-bridge circuit are known for generating an alternating voltage from a direct voltage, which for example provide a potential-free direct voltage in switched-mode power supplies in combination with a downstream transformer having a rectifier.
Three-phase or multiphase bridge circuits are also known for generating a multiphase alternating voltage, as are provided for example in frequency converters for the variable rotational speed adjustment of alternating current motors. Such converters for supplying power to electric motors are known to supply power to each motor supply line via a half-bridge of semiconductor switches, the half-bridges being supplied from a unipolar voltage, the so-called intermediate circuit voltage. The intermediate circuit voltage is produced by a mains-fed rectifier or, when the electric motor is operated as a generator, by the electric motor itself, and reaches values of 500 volts and more.
The semiconductor switches in such configurations are operated in pulse-width modulated fashion. In the simplest case, the semiconductor switches are “hard-” switched. In the switching process, a current then simultaneously flows through the semiconductor switch, while a voltage drops on it as well. The semiconductor switch is so to speak operated in its active range for the duration of the switching process, which can result in significant switching losses.
The magnitude of the switching losses is also influenced by the behavior of free-wheeling diodes provided in the half-bridges in parallel to the semiconductor switches. In MOSFET semiconductor switches, these diodes are intrinsically present and have great reverse currents and a correspondingly high reverse current chopping such that in systems that use these intrinsic diodes as free-wheeling diodes, high switching losses and transient overvoltages on the semiconductor switches occur.
In so-called DC-DC converters, as described, for example, in U.S. Pat. No. 6,356,462, resonant technologies are used, which allow for a switching at zero voltage, which may be called ZERO VOLTAGE SWITCHING, or at zero current, which may be called ZERO CURRENT SWITCHING. The multitude of specific embodiments may be grouped into resonance converters and quasi resonance converters, where in the quasi resonance converter, in which a soft switching may be performed, the resonance is essentially manifested only for the period of the switching process. Compared to the hard-switched converters, a higher switching expenditure is required in the resonance converters. Moreover, the respective circuit topology must be taken into account in the design, particularly also that of the connected load. Often restrictions apply regarding the control methods.
PCT International Published Patent Application No. WO 00/16407 describes a special semiconductor switch, the drain-source capacitance of which is extremely voltage-dependent. At voltages lower than approximately 10% of the operational switching voltage, the drain-source capacitance has a very high value, which, however, with a rising voltage quickly diminishes to a very low value. With this property it is possible to achieve a low-loss switch-off since the current very quickly turns from the channel current to the charging current of the drain-source capacitance.
U.S. Pat. No. 4,841,166 describes a gate control circuit for reducing overvoltages when using MOSFET semiconductor switches and their intrinsic diodes as free-wheeling diodes. In this instance, when switching on a transistor, the current through the source terminal is detected and, if necessary, the switching process is slowed down by reducing the gate voltage such that the reverse current through the free-wheeling diode and the resulting overvoltage are limited. However, the switch-on losses are increased compared to a circuit design having a fast recovery diode instead of the intrinsic diodes of the MOSFET semiconductor switches. This document cites as related art a circuit configuration in which the one first diode optimized for switching applications is connected as a free-wheeling diode antiparallel to the series connection of a MOSFET semiconductor switch with a second diode. The second diode has the effect that the free-wheeling current necessarily flows via the optimized free-wheeling diode and not via the intrinsic diode.